Infrared (IR) detectors are often utilized to detect fires, overheating machinery, planes, vehicles, people, and any other objects that emit thermal radiation. Infrared detectors are unaffected by ambient light conditions or particulate matter in the air such as smoke or fog. Thus, infrared detectors have potential use in night vision and when poor vision conditions exist, such as when normal vision is obscured by smoke or fog. IR detectors are also used in non-imaging applications such as radiometers, gas detectors, and other IR sensors.
Infrared detectors generally operate by detecting the differences in thermal radiance of various objects in a scene. That difference is converted into an electrical signal which is then processed. Microbolometers are infrared radiation detector elements that are fabricated on a substrate material using traditional integrated circuit fabrication techniques. Microbolometer detector arrays consist of thin, low thermal mass, thermally isolated, temperature-dependent resistive membrane structures. They are suspended over silicon readout integrated circuit (ROIC) wafers by long thermal isolation legs in a resonant absorbing quarter-wave cavity design.
Conventional infrared detector arrays and imagers operating at ambient temperature include microbolometer arrays made of thin films of hydrogenated amorphous silicon (a-Si:H) or amorphous vandium oxide (VOx). Other materials used for microbolometer arrays include films of various metal (e.g., titanium) and high temperature superconductors. For an array based on amorphous silicon, the detector pixel membrane is generally comprised of an ultra-thin (˜2000 Å) a-SiNx/a-Si:H/a-SiNx structure. The membrane is deposited at a low temperature nominally below 400° C. using silane (SiH4) and ammonia (NH3) precursors for the amorphous silicon nitride (a-SiNx) layers, and using silane for the hydrogenated amorphous silicon (a-Si:H) layer. Hydrogen atoms from silane (SiH4) molecules are the source of hydrogen content in the a-Si:H layer. A thin absorbing metal layer such as Titanium (Ti), Titanium-Aluminum alloy (TiAl), Nichrome (NiCr), black gold, or other material absorbing in the infrared band of interest, (e.g., at wavelength range of 1 micron to 14 micron), is inserted in the membrane to enhance infrared absorptance. Contact between the a-Si:H detector electrodes and the interconnect pads on a complementary metal oxide semiconductor (CMOS) signal processor of the ROIC is accomplished by thick aluminum tab metal interconnects.
After fabrication, microbolometers are generally placed in vacuum packages to provide an optimal environment for the sensing device. Conventional microbolometers measure the change in resistance of a detector element after the microbolometer is exposed to thermal radiation. Microbolometers have applications in gas detectors, night vision, and many other situations.
The primary factors affecting response time and sensitivity of microbolometers are thermal mass and thermal isolation. Microbolometer response time is the time necessary for a detector element to absorb sufficient infrared radiation to alter an electrical property, such as resistance, of the detector element and to dissipate the heat resulting from the absorption of the infrared radiation. Microbolometer sensitivity is determined by the amount of infrared radiation required to cause a sufficient change in an electrical property of the microbolometer detector element. Microbolometer response time is inversely proportional to both thermal mass and thermal isolation. Thus, as thermal mass increases, response time becomes slower since more infrared energy is needed to sufficiently heat the additional thermal mass in order to obtain a measurable change in an electrical property of the microbolometer detector element. As thermal isolation increases, response time becomes slower since a longer period of time is necessary to dissipate the heat resulting from the absorption of the infrared radiation. Microbolometer operating frequency is inversely proportional to response time. However, microbolometer sensitivity is proportional to thermal isolation. Therefore, if a specific application requires high sensitivity and does not require high operating frequency, the microbolometer would have maximum thermal isolation and minimal thermal mass. If an application requires a higher operating frequency, a faster microbolometer may be obtained by reducing the thermal isolation which will also result in a reduction in sensitivity.